This specification is concerned with equalisation in the sense of the compensation of amplitude and group delay time distortion of transmitted signals introduced by the transmission channel. This equalisation technique is mainly used for digital transmission over telephone circuits. Equalisation is effected by connecting at the reception end of the transmission channel a correcting circuit known as an equaliser with a transfer function which is as close as possible to the reciprocal of that of the transmission channel, so that the overall response characteristic is linear in phase and amplitude over the frequency band occupied by the transmitted signals. The correction circuit is generally self-adapting, its transfer function changing with time to take account of variations in the characteristics of the transmission channel. These circuits often use transversal filters in which the overall response of a filter element is synthesized as the weighted sum of the partial responses obtained at intermediate taps of a cascade-connected impedance array. The self-adapting function is obtained by causing the weighting coefficients to converge to optimum values for which the equaliser output signal bears the closest relationship to the transmitted signal. Convergence is obtained by means of an error signal based on various criteria well known to those skilled in the art, in particular the forced zero, hybrid and least mean-squared error criteria. The error signal must be representative of the differences between the waveforms of the transmitted signal available at the output of the equaliser and the estimated or known waveform of the signal at the transmission end. One method of generating the error signal consists in deducing the distortion imposed during transmission by means of a control signal which is superposed on the useful signal and of a form which is known at the reception end.
The present invention relates to a self-adapting line equaliser using transversal filters with controlled weighting coefficients, for use on a transmission channel carrying a useful signal and a superposed control signal.
A prior art filter of this type comprises:
a local generator supplying a control signal identical to that applied to the transmission channel, synchronized with the control signal received over the transmission channel and mixed with the useful signal,
a first transversal filter with controlled coefficients synthesizing the transfer function of the transmission channel on the basis of a comparison of the locally generated control signal and the signal obtained from the transmission channel, the first transverse filter also separating the useful signal from the control signal,
a second transversal filter with controlled coefficients synthesizing the reciprocal of the transfer function of the transmission channel on the basis of a comparison of the output signal of the first transversal filter and the locally generated control signal, and
a third transversal filter with the same coefficients as the second and having its input connected to receive the error signal of the first transversal filter.
A disadvantage of this type of equaliser is its relative complexity, the equaliser comprising three transversal filters. The need for a high signal/noise ratio for the useful signal at the output of the first transversal filter (for example, not less than 40 dB) means that the transmission level of the control signal must be very much lower than that of the useful signal. This makes it difficult to synchronize the local control signal generator.
The present invention is intended to provide a self-adapting equaliser of simple design operating with a control signal input level comparable with that of the useful signal whilst supplying at its output a useful signal in which the noise content due to the control signal is low.